Graphene has a two-dimensional carbon nano structure recently discovered and is formed in a single planar sheet of sp2-bonded carbon atoms in a honeycomb crystal lattice. A shape of the graphene is the same as that of fully exfoliated graphite. Graphite has a laminated structure of honeycomb crystal lattices.
In 2004, Professor Andre Geim et al. from the University of Manchester first mechanically exfoliated graphene from graphite by using “Scotch tape method” and found excellent electric conductivity of graphene through the study of a quantum hall effect by using the exfoliated graphene. Longitudinal scission of a carbon nanotube makes a graphene structure and infinite enlargement of a wall diameter makes a carbon nanotube similar to graphene. Therefore, electrical, thermal, and mechanical properties of graphene are expected to be comparable to those of carbon nanotubes. In 2008, James Hone et al., researchers from Colombia University, confirmed superior strength of graphene. In the same year, Alexander Balandin et al., researchers from University of California, Riverside, measured thermal conductivity of graphene as 5300 pW/mpK, which is double that of carbon nanotubes.
For preparation of graphene, mechanical exfoliation of graphite crystals as carried out by Professor Andre Geim et al., epitaxial growth on substrates, hydrazine reduction on graphite oxide sheet, chemical vapor deposition, and cutting nanotubes in a solution of potassium permanganate and sulfuric acid have been known but none of them go beyond laboratory preparation levels.
On the other hand, a method of producing expanded graphite, a shape of which is worm-like or accordion-like, by intercalating graphite flakes between graphite crystal layers by adding acids to the graphite flakes and adding a thermal shock thereto has been known since long before. Such worm-like expanded graphite is used as a filler or compression-processed to be used as a sheet having anisotropic conductivity. Such expanded graphite resulting from loose bonding between layers of part of graphite is inferior to graphene in physical properties and its particulate size is much bigger than graphene.
As a method for preparing graphite oxide, there has been known a Staudenmaier method in which graphite powder reacts with a sulfuric acid, a fuming nitric acid, and potassium perchlorate for days to produce graphite oxide. Further, it is described in U.S. Pat. No. 2,798,878 that Hummers shortened a reaction time by using a sulfuric acid, sodium nitrate, and potassium permanganate as oxidants. A reaction of mixing a sulfuric acid, sodium nitrate, and potassium permanganate together is an exothermic reaction and during the mixing reaction, Mn2O7 produced by a reaction between a sulfuric acid and potassium permanganate may cause explosion at about 55° C. or more. Thus, in this method, graphite oxide in a very small amount can be prepared only by a batch process and there is a limit on mass production of graphite oxide. However, in Korean Patent Application No. 2010-76871, some of the present inventors have suggested a method for continuously preparing graphite oxide using a micro reactor in which multiple micro channels each having a diameter of several μm to several mm and with a residence time of several minutes to several days are connected to each other. Further, in Korean Patent Application No. 2011-53777 which is a pending application filed by the present inventors and has not yet been laid open, there have been suggested a preparing apparatus with improved efficiency in which a tube reactor using ultrasonic waves is combined with a continuous stirred tank reactor.
In Korean Patent Application No. 2010-76871 and its Divisional Application No. 2011-24855, the present inventors describes an apparatus in which a prepared graphite oxide is dropped in a vertical fluidized bed furnace and floating graphene is separately collected by a cyclone. However, in such an apparatus, a drop of graphite oxide and a rise of graphene are carried out concurrently, and, thus, it is difficult to uniformly apply heat with low efficiency.